Hydroforming, a process by which extremely high internal fluid pressure is used to expand the wall of an unshaped metal blank out into the inner surfaces of a shaped die cavity, is now extensively used to produce beam like, hollow structures. Examples include auto body frame rails and the like, in which a cylindrical blank is clamped between a pair of dies, plugged at the ends, and expanded out into a generally square or rectangular cross section. Only the ends of the blank need be sealed, which are relatively small and uniform in shape. Such plug type seals also provide an obvious point for pumping in (and draining out) the high pressure fluid, through inlet pipes passing through the center of the plug seals. Recent patents in the area are mostly variations of or additions to the basic beam forming process, in which holes can be simultaneously pierced through the beam wall, or flanges incorporated, or varying wall thicknesses produced. Generally, however, the part produced is still an elongated beam, whose surface area is very small relative to its length.
Less attention has been paid to hydroforming as a means to produce large surface area sheet metal auto body parts, such as hood and deck lids. Clearly, the sealing task involved would be much more difficult than merely plugging two small ends of a cylindrical tube blank. The entire perimeter edge of the sheet would have to be sealed. There are known methods and presses for hydroforming single layers of sheet metal. An example is disclosed in U.S. Pat. No. 5,372,026 issued Dec. 13, 1994 to Roper. Sealing is accomplished by tightly clamping the entire perimeter of the single sheet between peripheral lock beads of a pair of upper and lower dies. An upper, shaped die provides the surface against which the sheet blank is expanded, the shape to which the sheet will ultimately conform. A lower die simply provides the other side of the pressurized die cavity and may be flat or, at least, devoid of particular surface detail. The task of expanding the single sheet is straightforward, once its perimeter edge is clamped and sealed by the lock beads. Pressurized fluid need only be inlet through a convenient port or ports in the lower die. Such fluid cannot escape past the clamped peripheral edge of the sheet, and can only act to expand the sheet into the shaped upper die. The inlet ports through the lower die need not be sealed in any way, and can be simple, open passageways.
Single layer, formed metal sheets would have limited utility in producing auto bodies, however. Almost all large surface area parts, such as hood and deck lids, are hollow, with inner and outer panels structurally joined at a peripheral seam, for rigidity and strength. Each panel could be separately formed as described above, and then later edge fastened to the other. However, this would obviously require two dedicated presses, and the final step of structurally edge joining the two separately formed panels would require that the two be very accurately positioned relative to one another, in yet another fixturing and joining apparatus. The most basic advantage of a hydroforming operation is typically to avoid such later fabricating steps, in which a final part is built up from separate components. This saves time and, most importantly, the accuracy and quality of a one piece hydroformed part is "built in" within the die, and repeatable. To first hydroform two large area sheet metal panels and then later structurally join them into a final part would probably not be considered cost effective, in most cases, as compared to simply stamping out the two pieces first. However, there is no known method for producing such a two paneled part entirely within the die, in one step, or, more importantly, to successfully introduce pressurized fluid into the interior. There is no obvious, workable location for the introduction of such fluid, as with the plug seals used in a cylindrical blank, where an inlet can simply run through the center of the plug, or as with hydroforming a single sheet, where the fluid is simply introduced to the back of the sheet with no need for sealing.